Glass-reinforced thermoplastic resin compositions based on synthetic linear polypyromellitimides



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tricity, or moisture.

GLASS-REINFORCED THERMOPLASTIC RESIN COMLPUSITIONS BASED ON SYNTHETICLIN- E POLYPYROMELLITMIDES Donald Lee Brebner, Claymont, and WalterMurray Edwards, Ivan Maxwell Robinson, Edward Norman Squire, and HowardWarner Starkweather, Jr., Wilmington, Del., assignors to E. I. du Pontde Nemours and Company, Wilmington, Del., ,a corporation of Delaware NoDrawing. Continuation of application Ser. No. 515,244, June 13, 1955.This application Oct. 7, 1957, Ser. No. 688,441 I 7 Claims. 7 (Cl.260-37) This invention relates to novel organic resin/glass compositionsand to mixtures useful in preparing them. The invention is especiallyconcerned with reinforced plastic'compositions having a combination ofdesirable properties including the capacity to be rapidly postformed,

lmown to be capable of fabrication by rapid postforming techniques, butin general have relatively poor resistance to either mechanicalstress,'high temperature, elec- In many cases they are also difficult toprepare.

It is ,a general object of the present invention to provide novelresin/glassjcompositions based on thermoplastic resins which manifestsurprisingly' high strength and resistance to boiling water, and may inaddition be readily postformable and possess. excellent high temperatureand electrical properties. A' further object is to provide novelmixtures which may be readily converted into compositions of this kind.Other objects will be apparent hereinafter. a

,According to the present invention it has been found that resin/ glasscompositions based on'cert-ain high-molecular-weightpolypyromellitimides manifest the sought-for It has further been foundpyromellitimides may be readily and rapidlyconverted into'reinforced'plastic compositions of this type. synthetic linear polyimides may beobtained by the condensation, polymerization of tetracarboxylic acids ortheir imideforrning derivatives with diprimary diamines having a radicallength of at least four, as disclosed in British Patent 570,858. In U.S.Patent 2,710,853, issued tov W. M. Edwards and I. M. Robinson, it isdisclosed that synthetic linear polypyromellitimides, based onpyromellitic acid and diprimary diamines of the group consisting of3-methy1 heptamethylene diamine, 4,4- dimethyl heptamethylene diamine,and nonamethylene diamine, have a unique combination of physicalproperties, especially at elevated temperatures. Thishigh molecularweight synthetic linear polypyromellitimides with which the presentinventionis concerned include those hereinabove mentioned as disclosedby Edwards and Robinson, and in general comprise high molecular weightsynthetic linear polypyromellitimides having low to -moder-atecrystallinity, a glassy statetransition temperature of at least 100 C.,'and a crystallinemelting point 2,944,093 Patented July 12, 1960 below400 C. Of these poly-4,4-dimethylheptamethylenepyromellitimide isespecially preferred.

High molecular weight synthetic linear polypyromellitimide, as usedherein, refers to a synthetic polypyromellitimide which is soluble inmeta-cresol, and which manifests an inherent viscosity of at least 0.4as measured at 0.5% concentration in meta-cresol at 25 C. Low tomoderate crystallinity refers to a degree of crystallinity in the rangeof 10 to 60% as determined by cooling a sample of polymer from above itscrystalline melting point to below C. during about 3 minutes,continuously recording an X-ray spectrometer diffraction pattern of thesample after it is cooled, and then comparing the crystalline andamorphous regions of the recorded patterns. Glassy state transitiontemperature" is defined in the aforementioned Edwards and Robinsonpatent, and refers generally to that temperature below the melting pointat which many of the physical properties of the polymer begin to changesharply. Crystalline melting point refers to temperature at whichsubstantially all of the X-ray patterns characteristic of crystallinitydisappear.

The polypyromellitimides in the aforesaid category have been found topossess the capacity to form surprisingly strong, moisture-resistantbonds with glass surfaces, by reason whereof resin/glass composites,having a plurality of glass surfaces bonded together with these resins,are also surprisingly strong and moisture-resistant. The high rigidityof these composites, is not affected by temperatures of 100 C, and theywithstand boiling water or steam without appreciable change in form orstrength. Nevertheless, being based on thermoplastic resins, which arecapable of flowing and rewelding to themselves at temperatures in therange of 300 to 400 C., they may be readily and rapidly postfo-rmed,without are of low to moderatecrystallinity makes it possible to obtaincomposites which have the capacity to retain very high percentagesoftheir rigidity at elevated temperatures, for example, up to 70% at 250C. They may also manifest excellent electrical properties.

A wide variety of polypyromellitimides in additionto those hereinaoovementioned, are suitable components of the compositions of the presentinvention. In addition to those hereinabove mentioned, these include,for example, homopolymers, such as those based on heptamethylenediamine, octamethylene diamine, or decamethylene diamine, andcopolymers, such asthose based on mixtures of nonamethylene diamine andhexamethylene diamine in mole ratios of at least 5 to 1, mixtures ofdi(paraaminocyclohexyl) methane and hexamethylenediamine in 7/1 moleratio, mixtures of nonarnethylene diamine and di(paraaminocyclohexyl)methane in mole ratio of at least 3/1, mixtures of 4,4-dimethy1heptamethylene diamine and di(paraaminocyclohexyl) methane in mole ratioof at least 1/2, and the like. As previously indicated the suitablepolymers are characterized by low to moderate crystallinity, glassystate transition temperature above 100 C, and crystalline melting pointbelow 400 C. In general composites based on polypyromellitimides of verylow crystallinity, such as the homopolymer derived from2,11-diaminododecane, invariably fall off very rapidly in strengthproperties at temperatures above their glassy state transitiontemperature. Composites based on polypyromellitimides having a glassyosa es a? state transition temperature below 100 C. such as thehomopolymers derived from l,2-bis-(3-aminopropoxy)- ethane do notpossess the-desirable property of being substantially unaffected byboiling water. Composites 'based on polypyromellitimides melting above400 C.

desirable.

The composites of the present invention may be prepared from any of theseveral forms of glass commonly used in preparing composite articles ofglass and organic plastics, including powders, fibers, filaments, yarns,

chopped fiber mats, woven fabrics, and solid plates. Choice of formdepends upon the particular characteristics desired in the ultimatearticle, in accordance with considerations herein disclosed, and otherswell known in the art. Composites having a least dimension not exceedingabout /s inch, based on fibrous glass, especially woven glass cloth, andparticularly, laminated structures, are preferred for maximum strengthand postformability. For improved retention of stiffness at elevatedtemperatures above 100' C., with the resins of relatively lowcrystallinity, such as poly-4,4-dimethyl heptamethylenepyromellitimide,the weight ratio of resin to glass fiber is preferably in the range of3/7 to 4/6, the radius of the fibrous glass is preferably at least0.0001 inch, and the length of the glass fibers is preferably at least0.125 inch. With resins of moderate crystallinity such aspolynonamethylene pyromellitimide, the weight ratio of resin to glassfiber may be 4/1 to 3/7, while the fiber should be at least 0.0001 in.in radius and at least 0.25 in. long. free and thoroughly dry beforeuse.

Preferably the high molecular weight resin/ glass composites of thepresent invention are substantially void-free structures in which allglass surfaces, not externally exposed, are thoroughly wetted by theresin since these exhibit maximum strength and minimum moistureabsorption. In some instances, adequate freedom from voids and wettingof the glass may be achieved by heating alternate layers of glass andfilms of resin, of inherent viscosity in the range of 0.4 to 0.8, underpressure. Alternatively, at somewhat greater expense and inconvenience,the glass surfaces can be coated with 'resin from solution inmeta-cresol or other effective solvent, and subsequently bonded withheat and pressure to obtain substantially void-free structures in whichthe glass is thoroughly wetted by the resin. The prior art, however,does not solve the problem of preparing such structures convenientlywhere condensation type polymers are involved. In general, condensationpolymers are too vis- In any case the glass surfaces should be greasecous when polymerized to high molecular weight to be a capable ofthoroughly wetting a plurality of surfaces of finely divided glass andflowing in to fill all voids. Polymerization of the resin from low tohigh molecular weight in the presence of the glass makes it difiicult toeliminate condensation by-products and hence leads to a bubbledstructure unless long times in expensive polymerization equipment areused.

Contrary to what would be expected from the prior 7 art, however, it hasbeen found that mixtures of glass and low molecular weight solid-stagecondensation products of organic salts of pyromellitic diesters anddipridue thermal degradation.

. 4 point of the dry organic salt, but is preferably methanol, ethanol,or propanol.

These mixtures may be prepared in various ways, conveniently by (l)impregnating the glass substrate with a solution of the organic salt ofthe diprimary diamine and pyromellitic diester, in a volatile solvent,(2) evaporating the solvent to deposit the salt on the glass surfaces ata temperature below about 138 C., which is the threshold temperatureabove which appreciable condensation polymerization commences, and (3)heating the resulting porous mass at a temperature above 138 C. butbelow the melting point of the resin-forming component, preferably inthe range of 138 to C. and ordinarily not above 200 'C., until aninherent viscosity in the range of 0.04 to 0.4 is achieved.

Various methods of preparing suitable organic salt solutions aredisclosedin the copending application of Edwards et al. (US. Ser. No.515,245), filed on June 13, 1955. A convenient procedure involveswarming pyromellitic anhydride with alcohol to obtain a solution ofdialkyl pyromellitate (the dialkyl ester of pyromellitic acid), and thenadding diamine to obtain a solution of polyalkylene diaminonium diethylpyromellitate. A slightmolar excess, forexample, 0.1 to 2%, of thediester may be used to function as a viscosity regulator and to minimizecolor development in the ultimate resin. Impregnation of the substrateglass with the salt solution and evaporation of the solvent attheprescribed temperatures deposits crystalline salt on the glass surfacesin the form of hue particles, leaving a porous mass. Preferably tominimize'color development, oxygen is excluded durporous mass,conveniently for an hour or more in a circulating air oven.

The resulting mixtures of glass and partly polymerized resin-formingmaterial may then be converted into substantially void-free. reinforcedplastic compositions by heating rapidly to elevated temperatures above300 C. and above the crystalline melting point of the resin beingformed, preferably under slight pressure, and subsequently causingtheresin to solidify. Heating may be carried out in various ways, usinghot inert gas, dielectric current, infrared radiation, or heated platensas a heat source. Preferably a mold release agent, such as a siliconeoil, is used to prevent the heated mass from sticking to its support."Alternatively the mixture may be supported on or between metal foilwhich is thereafter stripped or dissolved away. The time required tocomplete the conversion depends upon factors such as the least dimensionof the mixture, the particular resin, the actual temperature, and therate of heat transfer achieved, but in general does not exceed fiveminutes and is ordinarily less than about two minutes, when the leastdimension of the mixture does not exceed about /s. 7

For best results, the thickness of the mixture, in the sense of leastdimension, is preferably as low as practicable, taking account of thefact that thicker articles may be prepared readily by laminating theconverted mixtures. The temperature used is preferably as far above themelting point of the resin as practicable without causing un- The rateof heating is also preferably as high as practicable, and pressure isused as necessary to take best advantage of the low viscosity of theresin at the onset of melting, to cause it to fill the void spaces andwet the glass surfaces.

It will be recognized that these mixtures of glass and resimformingcomponents also possess a unique combination of'properties. Thus theymay be preformed and heated to transform them into ultimate articles ina manner similarto but ordinarily quicker than that used with mixturesbased on thermosetting resin intermediates, but being thermoplastic theyyield products which may be reheated and joined to themselves or otherarticles to form butt weided orlaminated articles.

Other materials in addition to the glass and resinforming component mayalso be included in these mixtures to confer special characteristics.upon the final products obtainable from them. Examples of such additivesinclude viscosity stabilizers, heat stabilizers, color stabilizers,pigments, dyes, and other fillers of metal, siliceous material,synthetic polymers and the like. Finely divided carbon of colloidal sizemay be incorporated into a solution of the organic salt previouslymentioned, in amounts of about 2 to by weight of the resin, to producean ultimate product having excellent weather resisting properties.Blowing agents may be similarly incorporated to produce foamedstructures in which the resin nevertheless thoroughly wets the glasswith which it is in contact. Finely divided polytetrafiuoroethylene maybe added to produce ultimate articles having improved bearing andanti-stick properties. Various other modifications will be apparent tothose skilled in the art.

The several aspects of the invention are more specifically illustratedby means of the following examples, which, however, are not intended'tolimit its scope.

Example I Pyromellitic anhydride is dissolved in absolute ethanol withgentle warming to obtain a solution of diethyl pyromellitic acid. Anequimolar amount of 4,4-dimethylheptamethylene diamine is slowly addedto this solution with constant stirring, and the resulting acid-diaminereaction product is isolated by low temperature evaporation of theethanol under reduced pressure. The isolated product is redissolved inan approximately equal weight of distilled water. The resulting solutionis used ,to impregnate six individual plies of glass cloth 181-112,which is described as a heat-cleaned satin-weave cloth especiallysuitable for the preparation of laminates. The impregnated sections aredried in an air oven for 2 hours at 100 C., and subsequently heated at140, C. for two hours. The plies are then individually pressed at lowpressure in a chase for 1.5 minutes at 340 C. and subsequently pressedtogether at 340 C. for two minutes to form a tough, pale yellow,substantially void-free 6-ply laminate containing about 34% resin byweight. A sample of the resin component has an inherent viscosity ofabout 0.5 as measured in meta-cresol at 25 C. A further sample of theresin component has a degree of crystallinity of about 10-20% asdetermined by analysis of X-ray spectrometer diffraction patterns.Average results of standard ASTM tests on the laminate are as follows:

At temperatures of 340-380 C., the laminate is readily postformed,deep-drawn, or welded to itself to form articles of complex shape. It isalso sufficiently malleable to be cold-formed to an appreciable degree.

From the foregoing values, in comparison to those reported in theliterature for similar reinforced plastic compositions based on glassand organic thermosetting resins (see for example Modern PlasticsEncyclopedia Properties Chart) it will be apparent that this laminatecompares favorably with respect to mechanical, electrical, andmoisture-resisting properties generally, and is outstanding withExample, II

The procedure of Example I is duplicated in essential details exceptthat nonamethylenediamine is used. The properties of the product aresimilar to those of the preceding example, except that the compressivestrength is about 11,000 p.s.i., the Izod impact strength is about 9 ft.lbs. per inch of notch, and the water absorption is about 0.7%. Ameasurement of crystallinity of the resin component by the X-rayspectrometer method indicates a crystallinity of about 50%.

Example Ill Poly-4,4-dimethylheptamethylene pyromellitimide having aninherent viscosity of about 0.6 in the form of'a 2-5 mil thick film isused to make an assembly of seven layers of film interspersed with 6layers of 181-112 glass cloth. The assembly is pressed at 1000 p.s.i. between sheets of aluminum foil for 3 minutes at 360-390 C. and allowed tocool. A laminate similar in strength properties to that of Example I isobtained, except that the compressive strength is about 15,000 p.s.i.and the moisture absorption is somewhat higher.

Example IV Poly-4,4-dimethylheptamethylene pyromellitimide having aninherent viscosity of about 0.9 is dissolved in hot metacresol. Theresulting solution-is used to coat'one surface of each .of two flatglass plates A; inch thick which have been carefully cleaned and heatedabove 150 C. The solvent is evaporated and the coated surfaces are thenpressed together in a press at low pressure and heated ,until excessresin flows out from between the plates, leaving a substantiallyvoid-free resin interlayer. The resulting sandwich is allowed to cool inair. The shear strength of the bond is determined by placing thesandwich in a slot between verticallypositioned metal blocks, the matingends of which form an angle of 45 to the horizontal. The lower block issupported on bearings such that it can move in the horizontal planeunder the impetus of a downward thrust. Pressure is applied to the upperblock and the bond fails at the glass to resin-surface at a shear stressof 5000 p.s.i.

Comparative tests on similarly prepared sandwiches With otherthermoplastic resins indicate shear strengths of 180 in the case ofpolystyrene, 1600 for polyvinyl butyral, 2500 forpolyhexamethyleneadipamide, and 2800 to 3200 for otherpolypyromellitimides of the group broadly defined hereinabove. Tests onsandwiches having interlayers of thermosetting resins range from about2500 for a cross-linked polymethacrylate to about 3700 p.s.i. forepoxide resins.

Example V A quantity of heat-cleaned glass fibers having a diameter ofabout 2 mils and an average length of about A is slurried with anaqueous solution prepared as described in Example I, and the resultingmixture is dried on a tray at C. for 2 hours and thereafter at 140 C.for two hours to yield a porous mat about A" in thickness. The mat ispressed at low pressure at 350 C. for 3 minutes and then allowed to coolto room temperature to obtain a thin tough composite containing about20% glass by weight. The fiexural modulus of the composite is 634,000 atroom temperature and 630,000 at C. A sample of the composite is boiledin warespect to retention of rigidity at elevated temperatures. 75 12 0Without ppreci ble ef ect On it properties. A sample boiled in mild soapsolution for five months loses 3% of its original weight but shows noother change. Samples submerged in ethanol, carbon tetrachloride,hexane, concentrated hydrochloric acid,

- turpentine, and linseed oil are unchanged after 800 hours at roomtemperature. Exposure of a sample to mixed steam and air for six monthscaused no change in color, weight or toughness. A sample shows no changeafter being heated for 350 hours in motor oil at 197 C.

The resin/glass composites of the present invention are useful in a widevariety of applications where their strength temperature resistance,moisture resistance, electrical properties, antifriction properties orready fabricability are advantageous. For example, they may be used asmaterials of construction for buildings, vehicles, cartons, machineparts, electrical insulation, electrical parts, bearings, slides,hotwater pipe, dining hall serving trays, etc. In both lowandhigh-molecular weight resin forms, the composites are readily fabricatedby matched-die molding. In the high-molecular weight forms they may alsobe sawed, stamped, punched, drilled and otherwise shaped by conventionalmethods, as well as deep-drawn, molded, welded, laminated, or otherwiseshaped while hot. Because the resin components are malleable,particularly as respects the composites based onpoly-4,4-dimethylheptamethylene pyromellitimide, they are capable ofbeing forced into tightly sealing relationship with themselves or othermaterials. Numerous oth er advantages and applications will be apparentto those skilled in the art.

We claim:

1. A glass-reinforcedthermoplastic resin composition, substantiallyunaffected in form and strength by boiling water-and having aresin-to-glass bond shear strength of at least 2800 lb./sq. inch,consisting essentially of reinforcing glass and synthetic linearpolypyromellitimide of low to moderate crystallinity having an inherentviscosity at 0.5 percentvconcentration and 25 C. in meta-cresol of atleast 0.4, a glassy-state transition temperature of at least C., acrystalline melting point not exceeding 400 C., and, as the solerecurring integral members of the main polymer chain, carbon andnitrogen bonded only to said carbon.

2. Composition of claim 1 inthe form of a sheet having a thickness notexceeding 0.125 inch.

3. Composition of claim 1 wherein the reinforcing glass is in fibrousform, present in resin-to-glass ratio of from 4/1 to3/7.

4. Composition of claim 3 wherein the fibrous glass is in the form ofwoven glass.

5. Composition of claim 1 wherein the polypyromellitimide ispolynonamethylene'pyromellitimide.

6. Composition of claim 1 wherein the polypyromellitimide ispoly-4,4-dimethylheptamethylene pyromellitimide 7. Composition of claim6 in the form of a sheet having a thickness not exceeding 0.125 inchwherein the glass is woven glass persent in glass-to-resin weight ratioin the range of 6/4 to 7/3.

References Cited in the file of this patent UNITED STATES PATENTS

1. A GLASS-REINFORCED THERMOPLASTIC RESIN COMPOSITION, SUBSTANTIALLYUNAFFECTED IN FORM AND STRENGTH BY BOILING WATER AND HAVING ARESIN-TO-GLASS BOND SHEAR STRENGTH OF AT LEAST 2800 LB./SQ. INCH,CONSISTING ESSENTIALLY OF REINFORCING GLASS AND SYNTHETIC LINEARPOLYPYROMELLITIMIDE OF LOW TO MODERATE CRYSTALLINITY HAVING AN INHERENTVISCOSITY AT 0.5 PERCENT CONCENTRATION AND 25*C. IN A META-CRESOL OF ATLEAST 0.4, A GLASSY-STATE TRANSITION TEMPERATURE OF AT LEAST 100*C., ACRYSTALLINE MELTING POINT NOT EXCEEDING 400*C., AND AS THE SOLERECURRING INTEGRAL MEMBERS OF THE MAIN POLYMER CHAIN, CARBON ANDNITROGEN BONDED ONLY TO SAID CARBON.